I need some advice to troubleshoot my in-boat audio system. Here are the details:

-I am running 6 speakers on a 4-channel head unit (Kenwood KDC-2019)
-The two back left speakers are connected in series
-All connections are good, speakers are good
-The two back left speakers stopped working all of a sudden. Sometimes they randomly come on for a few seconds
-Neither of the speakers are getting any voltage/current from the head unit when playing music

I would do more troubleshooting to get more info today, but we got some severe weather here.

I'd appreciate any pointers on where to look. I'm thinking I might reconfigure the wiring so that the head unit powers just four speakers and two of them are on one of my amps. The boat came wired like this, and I'm not sure if it's the best setup. If the head unit went bad, I might get another one, considering this one is fairly old.

6 speakers on a 4 chnl amp is perfectly fine and far better then moving any to the head-unit. I would however, confirm whether the 2 aft pair are wired in series or parallel. If the speakers are 4 ohm speakers and the amp is 2 ohm stable x 4, then parallel would be the better configuration.

A good visual and psychical inspection of the speaker wire connection at the amp and at the speakers is a good place to start. Next, check the RCA cables and you can even swap the left and right cables to the see if the problem stays or moves. A digital volt/ohm meter will allow you to test the suspect speaker circuit at the amp. From there, you can back track toward the speakers until you isolate the problem.

Just as clarification, they are all on the head unit (4x50 W) and none are on the amp. I'm pretty certain they are in series because when I unplugged the first speaker, the wires to the second speaker didn't form any circuit (I've checked all with digital multimeter).

I'm not sure if my head unit is 2 ohm stable. That's why I was thinking of putting my 2 front speakers on an amp and the 4 aft speakers on the head unit.

The Kenwood HU is definitely NOT 2-ohm stable. Unfortunately many of the factory stereos wired two 4-ohm speakers in parallel for a 2-ohm load. The unstable HU would really fall on its face prematurely. Although you wouldn't anticipate this, everything sounded infinitely better and played louder when the speakers were wired in series for an 8-ohm load. The 2-ohm load would cause the HU to intermittently cut out when pushed and would eventaully toast the HU.
Make absolutely certain that the speakers are in series rather than parallel. The DCR of the parallel circuit would measure around 1.75 to 2-ohms. The DCR of the series circuit would measure around 7 to 8-ohms. I would verify this. I would also test each of the speakers in question individually on an alternate channel. Then test the HU channel in question with a different speaker at the HU and bypassing the in-boat wire.